Heretofore, in order to mark or delineate for visibility at night time by vehicle drivers road side edge portions (or road side edge barriers, or road median strips delineating the space interval between two adjacent highway surfaces running parallelly to one another, or curved exit or entrance ramps associated with so-called super highways and other roadways, or tunnels, or bridge structures, or any harzardous object along a road), it has been conventional to employ vertically oriented reflectors. In general, three types of prior art reflectors have been employed for this class of application: (1) the prismatic triple mirror type reflector using a plurality of standard reflex type units wherein the individual cube axes are perpendicular to the surface of the reflector; (2) the glass bead-type reflector sheeting wherein glass beads are embedded thereinto; and (3) the prismatic triple mirror type reflector using a plurality of angled cube corner-type retroreflective units (such as a reflector adapted from the teachings of Heenan U.S. Pat. No. 3,332,327) which is normally mounted horizontally on the pavement as a center line marker or the like, but which in this application is mounted vertically and which here employs only the front ramp-like surface of such reflector body). In this class of application, the performance of all three of these reflectors is similar to one another in that the peak of retroreflectivity of each type in terms of light intensity when such is so mounted upon a road side edge or the like in a vertical orientation is parallel to a tangent to the road at that point. Retroreflectivity extends from that parallel position (sometimes termed 0.degree.), or from a peak intensity location, through typically angles up to about 25.degree. to 30.degree. into the roadway, although the glass bead-type reflector sheeting appears to have slightly more angular range than this, going up to perhaps about 40.degree..
In all known such prior art types of reflectors adapted for this class of application, the reflected light output, or performance of retroreflectivity, decreases with increasing angles to such tangent to the road. This decrease is such that, as a car moves down a roadway at night, for example, the driver has each individual road side reflector in view (as respects retroreflected light) only through a maximum angle typically not greater than about 30.degree.. For the rest of the time interval that the driver is before and approaching an individual reflector (which thus covers an angular range or zone of from about 30.degree. to 90.degree.), the driver is unable to see the individual reflector by retroreflected light because such reflector is not retroreflective of the car headlights in that zone. Perhaps the driver can physically see an individual such reflector, but once the driver is beyond 30.degree., and in the range of from 30.degree. to 90.degree., he can not see or receive any retroreflected light signal therefrom.
This limitation on retroreflective viewability causes serious night-time motorist hazards espcially on curved roads since a driver is unable to see a curve ahead, or a significant distance along a curve ahead, as he proceeds to drive along a road. What occurs is that the driver's line of sight as he proceeds to drive along the road enters the non-reflected zone in a very brief distance since the tangent to the curve in the road curve inherently has a continuously greater angle with increases in road curvature. Therefore, as the driver proceeds along a roadway which has an increasing degree of curvature with distance along the road, the driver has increasingly less delineation of the highway ahead owing to the inherent limitations of such prior art retroreflectors used for road edge delineation. This limitation is most particularly evident on exit and entrance ramps of limited access roads which ramps are extremely curved in relation to the normal road pathway connected with such a ramp.
For example, in a modern cloverleaf road interchange, where a main highway may over-cross an intersecting road via an overpass, the intersecting road is connected with the main road through a system of connecting roads called a cloverleaf. Thus, at the overpass, the main highway employs a bridge-type construction. Normally, on such a bridge-type construction, the median portion between the two oppposing directions of traffic on the main highway is an upright construction or supporting structure presenting a potentially dangerous hazard to a car which is entering the main highway from the intersecting road via one of the cloverleaf entrance roads. By nature of the shape of the individual cloverleaf road ramps, an entering car approaches the hazardous median at an angle which typically may be between 45` and 70.degree.. When delineated by such prior art reflector devices, the median reflectors are visible in advance to drivers proceeding in either direction along the main highway, but the median is not visible to the drivers approaching and entering into the main roadway from a cloverleaf road ramp, since, to such entering drivers, the angle of viewing of the median reflectors is beyond the retroreflectivity characteristics and capabilities of the median reflectors.
For another example, a similar situation exists with respect to cars exiting from such a main highway onto such a cloverleaf road ramp so as to enter upon such an intersecting road. In this case, the degree of curvature of the cloverleaf road ramp is so great that the prior art reflectors, when duly mounted so that the peak of retroreflectivity is parallel to the tangent to the curve of the road ramp, have a retroreflective viewing angle which increases rapidly with the curvature of the road and therefore permits a driver entering or on a road ramp to see simultaneously only a very limited number of such prior art reflectors for delineating the curve ahead since the driver's line of sight is beyond the zone of retroreflectivity associated with individual reflectors spaced and mounted along the roadway edge of the ramp.
From such examples, it is seen that there is a need for an improved roadside reflector system which will permit the drivers of vehicles to see individual roadside reflectors for a considerable distance along a road ahead. Such retroreflective viewability requires individual reflectors each of whose retroreflectivity characteristics extend from about 0.degree. up to at least about 60.degree. and these reflectors are then stationed at desired intervals along road side edge portions.
So far as is known, nothing in the prior art in any way teaches or suggests roadway edge marking reflector systems adapted to provide appreciable, or practically sufficient, retroreflectivity beyond about 30.degree. so that the zone of from 30.degree. to 60.degree. is actually completely uncovered by prior art roadside reflector systems. As indicated, between about 30.degree. and 40.degree., some retroreflectivity is provided by glass beaded reflectors (for example, a glass beaded sheet the so-called "Scotch-Light" (trademark) type available from Minnesota Mining & Manufacturing Company, St. Paul, Minn., but the 10.degree. wide zone from 30.degree. to 40.degree. is found to be only weakly retroreflective for such glass beaded sheeting so that the viewability and the retroreflectivity characteristics of glass beaded sheeting is generally considered by those skilled in the art of highway marking to be insufficient for adequate highway safety practices at these angles of from 30.degree. to 40.degree.. Currently, glass beaded reflectors are accepted as a 0.degree. to about 25.degree. material, and very little use is made of the retroreflectivity characteristics of glass beads in the range of from about 25.degree. to 40.degree. because of this inherent weakness. Thus, no known roads have ever been equipped with reflector constructions at spaced intervals along roadside edge portions such that individual reflectors provided vehicle drivers moving along the road with continuous reflectivity, from individual reflectors ranging at least from 0.degree. to 60.degree..
Consequently, there is a great need in the field of road safety for a barrier marker system in which individual reflectors retroreflect through angles of from at least about 0.degree. to at least about 60.degree. and preferably from about minus 5.degree. to plus 75.degree. relating to a road in order to cover the various approach angles inherently associated with vehicular operation along roadways at night time.
While, as indicated, no known individual reflector constructions adapted for this class of application have continuous retroreflective viewability characteristics over such ranges (relative to a road), cube corner type retroreflector constructions having retroreflective continuous viewability characteristics over a range of from about 0.degree. to 70.degree. (measured in the same relative direction as that herein used in reference to the present class of application) have heretofore been known to the prior art, but have been employed in, and developed for, other fields of application. For examples, see Heenan et al. U.S. Pat. No. 3,887,268, Heenan et al. U.S. Pat. No. 3,541,606; Golden et al. U.S. Pat. No. 3,887,268; Nagel U.S. Pat. No. 3,893,747; Nagel U.S. Pat. No. 3,894,786; Golden et al. U.S. Pat. No. 3,894,790; Nagel U.S. Pat. No. 3,895,855; Nagel U.S. Pat. No. 3,905,680; Nagel U.S. Pat. No. 3,905,681; and the like. Commonly even when prior reflectors have retroreflective capability through includes angles greater .+-.30.degree. such are not adapted for use in the highway marking field. For one thing, such prior art reflectors could have reflective characteristics which go so far beyond an included angle of 90.degree. that they become safety hazzards in that they could equally guide motorists approaching the same point from opposed directions. Observe also that, for example, even if one endeavours to move a reflector of the type shown in Heenan U.S. Pat. No. 3,332,327 to an elevated position along the side of a road, one still does not obtain continuous retroreflectivity from such a reflector through an angle of from 0.degree. to 60.degree.. In the highway barrier marker field, reflector constructions specially adapted for positioning and mounting along road side edge portions are needed and necessary in order to permit economical installation, low maintenance costs, long life, good reflectance characteristics over the ranges desired (as above indicated), and the like. New and improved reflector constructions which are specially adapted for this class of application are thus needed.